Method of improving ABS control behavior

ABSTRACT

In order to improve the control behavior of an ABS control system wherein, instead of the standard control mode, a special control mode or rather corner control mode comes into operation when a cornering situation is identified, said special control mode or corner control mode causing a reduction of the braking pressure on the front wheel and/or on the rear wheel on the inside of the corner, the gradient of the braking pressure build-up on the wheel on the inside of the corner is reduced in the partial braking range when a threshold value (th 1 , th 2 ) of a wheel speed difference (SD) is exceeded, which difference is the difference between the wheel speed sums per side formed from the wheel speeds of the two wheels of one side of the vehicle, the reduction taking place in dependence on the extent by which the threshold value (th 1 , th 2 ) is exceeded.

BACKGROUND OF THE INVENTION

This invention relates to a method for improving the control behavior ofan anti-lock control system and more particularly for improving thesteerability of the vehicle and the driving stability during cornering,wherein a vehicle reference speed is derived and criteria foridentifying a cornering situation and the direction of cornering areobtained from the rotational behavior of the vehicle wheels wherein,instead of the standard control mode, a special control mode or rather acorner control mode starts operating or is put into operation,respectively, when a cornering situation is identified, said specialcontrol mode or cornering control mode producing a reduction of thebraking pressure on the front wheel on the inside of the corner and/oron the rear wheel on the inside of the corner already in the partialbraking range.

In a cornering identification system, with a method of control of thetype presently mentioned, the pressure relief of the wheels on theinside of a corner generates a yawing moment around the vertical axis ofthe vehicle which balances and stabilizes the cornering situation. Withcontrol coming on during a partial braking operation, the brakingpressure on the wheels on the outside of the corner will be keptconstant or will automatically increase due to the cut-off of anyfurther build-up of braking pressure on the wheel or wheels on theinside of the corner.

From DE 34 13 738 C2 (P 5547) there is already known an anti-lockcontrol system (ABS) with a cornering identification circuit likewisebased on wheel slip measurement. For the purpose of identifying acornering situation the slip values of the wheels of one vehicle sideare added up and compared with the slip sum of the wheels of the othervehicle side and a cornering identification signal will be generated assoon as the difference between the slip sums exceeds a predeterminedlimit value. Selection criteria such as “select low” or “select high”criteria, according to which the pressure variation is controlled in theindividual braking pressure control channels of this brake system, andlimit values for the coming-on of these selection criteria will bevaried when a cornering situation is identified. In this way, control isto be adapted to the varying conditions during straight-onwards drivingand during cornering.

It is known from older DE 21 19 590 A1 to obtain a corneringidentification signal by means of a transverse acceleration measuringdevice such as a mercury switch.

It is further already known to expand the functions of an ABS controlsystem in that the system is used for improving the driving and brakingstabilities in corners. This is done in that a stabilizing moment isgenerated around the vertical axis of the vehicle by means of acalculated deceleration of the build-up of the braking pressure on thewheels on the inside of the corner as compared with the braking pressureon the wheels on the outside of the corner during cornering and apartial braking operation, i.e. during a braking operation, where ABSresponse threshold values are not reached (“Bremsanlage undSchlupf-Regelsystem der neuen 7er-Reihe von BMW”, ATZ 97 (1995), pages8-15; and “Bremsanlage und Schlupf-Regelungssysteme der neuen Baureihe 5von BMW”, ATZ 98 (1996), pages 188-194 [“Brake system and slip controlsystem of BMW's new no. 7 line of models {ATZ auto journal, 97 (1995),pages 8-15}; and “Brake system and slip control systems of BMW's new no.5 line of models” {ATZ auto journal, 98 (1996), pages 188-194}]). Withno steering angle sensor being used, the information on the currentsteering angle is derived from the transverse acceleration which, on itspart, is calculated from the wheel sensor signals.

It is thus an object of the present invention to develop a method of thetype referred to above which will provide a marked contribution toimproving the driving behavior and stabilization, respectively, of thevehicle by means of reliable cornering identification and reaction tothis situation.

SUMMARY OF THE INVENTION

It has been found out that this object is achieved by a method theparticular features of which consist in that the gradient of the brakingpressure build-up on the front wheel and/or on the rear wheel on theinside of the corner is reduced in the partial braking range when athreshold value of a wheel speed difference is exceeded, whichdifference is the difference between the wheel speed sums per sideformed from the speeds of the two wheels of one side of the vehicle, thereduction taking place in dependence on the extent by which thethreshold value is exceeded.

In practice, pressure management of the front wheels is of particularimportance in the special control mode or rather in the corner controlmode. As a rule, while reducing the braking pressure on the wheel on theinside of the corner it is beneficial to simultaneously increase thebraking pressure on the wheel on the outside of the corner.

Thus, according to this invention, a wheel speed difference thresholdvalue is used for identifying a cornering situation and for calculatingand reducing, respectively, the further pressure build-up on the wheelon the inside of the corner the amount of which value, according to apreferred example of an embodiment of this invention, is varied independence on the vehicle speed. Below the threshold value, thereapplies the maximum value of the braking pressure build-up gradient. Theextent by which the threshold value is exceeded determines the reductionof the gradient which may fall from the original maximum value down to avery low value or, practically, may sink down to the value of zero.

According to another example of an embodiment of this invention, thereduction of the braking pressure gradient is adjusted by going over toa pulse-shaped introduction of braking pressure and by varying thepulse-and-pulse-pause ratio by means of which an inlet valve will beactuated.

Thus, this invention relates to the braking pressure correction on thewheel on the inside of the corner, more particularly on the front wheel,in order to enhance the steerability and driving stability duringcornering for the purpose of increasing the stability of the vehicle andimproving the braking stability under transverse-dynamic influences inthe partial braking range. There will be generated a stabilizing momentaround the vertical axis of the automotive vehicle.

Any further details and applications of this invention will becomeevident from the following description of examples of embodiments,reference being made to the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a block diagram of the essential electronic components orfunctional blocks of an ABS control system that is expanded forimplementing the inventive method;

FIG. 2 is a flow chart of the individual steps of the program run or ofthe logical operations of the inventive method;

FIG. 3 is a graph of the difference threshold value in dependence on the(reference) speed of the vehicle; and

FIG. 4 is likewise a graph of examples of predetermined pressurebuild-up gradients.

DETAILED DESCRIPTION OF THE DRAWINGS

The block diagram is schematically simplified as are the flow chart andthe graphs in order to illustrate what is essential in this invention.

FIG. 1 represents the fundamental mode of operation of a circuitconfiguration for anti-lock control expanded by components for improvingthe control behavior during cornering.

The input information of the control system is obtained by means ofwheel sensors S1 through S4. To this end, the individual(non-illustrated) vehicle wheels are equipped with transmitters ortoothed disks 1-4 rotating with the vehicle wheels and generating anoutput signal in the transducers, i.e., the stationary components ofsensors S1-S4. These output signals represent the rotational behavior ofthe individual wheels in a manner known per se.

After processing the sensor output signals in a circuit 5, there areavailable the wheel speed signals v₁-v₄. By means of logical combinationof the output signals of circuit 5, namely of the speed signals v₁-v₄,in a linking circuit 6, a reference speed v_(Ref) of the vehicle isformed which, above all, serves as a reference value for determining thewheel slip 8 ₁ through 8 ₄ of the individual vehicle wheels and, hence,also as a reference value for braking pressure modulation.

A circuitry block 7 contains the individual circuits for determining thewheel slip 8 ₁ through 8 ₄ by comparing the reference speed of thevehicle with the respective wheel speed v₁-v₄. As is known, the wheelslip results from the difference

8 _(i)=v_(Ref)−v_(i), with i=1 . . . 4.

With a program-controlled circuitry being used such as a microcomputeror microcontroller, block 7 symbolizes the program steps for calculatingthe slip 8 _(i) of the individual vehicle wheels.

In a circuitry block 8, each slip signal passes a low-pass filter thefiltering time constant of which may lie in the order of magnitude of50-200 msecs; in our example the time constant lies at 70 msecs. Thecircuitry block 8 transmits a filtered wheel slip signal fws_(i) andpasses this output signal on to a corner identification circuit 9,respectively.

In the corner or cornering identification circuit 9, the current and thefiltered wheel slip signals are analyzed and evaluated, with corneringidentification taking place in accordance with qualitative andquantitative criteria. The direction of the corner is likewisedetermined by evaluating and logically combining the slip signals andthe slip difference signals. The result of this slip evaluation by meansof circuit 9 then leads to the adaptation of anti-lock control to theparticular conditions of cornering, this being done via an additionalcircuit 10, namely a circuit in addition to the actual standard ABScontrol logic 11. Now, a special control or corner control will takeplace instead of standard control.

The output of the control logic 11 leads to the actuators or modulators12 via non-represented processing and evaluating circuits, the brakingpressure of a brake system being managed as required by means of theseactuators or modulators 12. In the present-day anti-lock controlsystems, the actuators 12 used are mainly electromagnetically actuatablehydraulic valves for modulating and controlling the braking pressure inthe individual wheel brakes.

Of course, the actual ABS control logic is likewise based on theevaluation of the processed wheel speed signals v₁-v₄, with thereference speed v_(Ref) of the vehicle being taken into consideration.

As already mentioned, the functions of a circuit configuration as perFIG. 1 can be realized by program-controlled circuits and by the programrun, respectively. Nowadays, such controller technology is preferred.Therefore, in the form of a flow chart, FIG. 2 shows the program runwhich transforms the inventive method into control steps. Thesefunctions, steps and logical operations are mainly sited in thecornering identification circuit 9.

FIG. 2 shows a detail of an ABS control program.

The inventive special program takes place outside actual anti-lockcontrol. If ABS control takes place (J=ja=YES) thus, following diamond13, standard control will continue. However, if the answer is “NO” (N),if thus ABS control is not in operation and if, according to branchpoint 14, there is indeed a braking operation (partial braking:J=ja=YES) the further process of decision will depend on whether or nota cornering situation was identified (15: YES). Now it must be found outwhether or not a predetermined wheel-speed-difference threshold value SD(branch point 16) was exceeded. This operation, or rather this thresholdvalue, will be explained in more detail in the following with referenceto FIG. 3.

If the threshold was exceeded the braking pressure build-up gradient ofthe wheel on the inside of the corner, in particular of the front wheelon the inside of the corner, will be adapted or rather varied incorrespondence with circumstances via the “YES”-output of the decisionpoint 16. This operation is symbolized by reference numeral 17 in FIG.2.

The braking pressure build-up gradient is generally expressed by theformula

p=f(th₁,th₂,th₃)

o<p<p_(max)

This equation will be explained in more detail with reference to FIG. 3.

If the threshold was not exceeded (16: N), the further program run willdepend, in accordance with decision point 18, on whether or not thespecial control mode already has led to a management of the brakingpressure reduction gradient. In case of “NO” (output N of branch point18), the program run will lead back to the starting point. In case of“YES” and if there was again a drop below the threshold valueth(th₁,th₂,th₃) and, additionally, below a tolerance value or ahysteresis value, i.e., below the value: “threshold minus hysteresis”(branch point 19: J=YES), the braking pressure build-up will becontinued with the maximum gradient p=p_(max) as symbolized by operation20 because the need for special control caused by cornering haspractically come to an end. A hysteresis value of 2 to 6 k.p.h. hasproved advantageous in one example of an embodiment of this invention.If there is a drop below the threshold value th by such an amount thiswill lead to the termination of special control. The hysteresis value isnot represented in FIG. 3.

If the answer to question 19 is “NO”, the braking pressure manipulationin functional block 21 will be performed in the same manner as in step17 (operator 17): The manipulation of the braking pressure build-upgradient or rather the reduction of the build-up gradient will becontinued in order to improve driving stability.

Thus, as explained with reference to the flow chart of FIG. 2, a partialbraking operation outside ABS control is a prerequisite of the inventivebraking pressure manipulation on the wheel on the inside of the corner.Of decisive importance is the slip or wheel-speed-difference thresholdwhich, according to FIG. 3, varies in dependence on the vehicle speed oron the reference speed of the vehicle. This wheel speed differencerepresents the difference between the slip sums per side formed from theslip values of the two wheels of one vehicle side. Expressed in aformula, this means:

SD=|(v_(VL)+v_(HL))−(v_(VR)+v_(HR))|

The indices VL, VR of this formula denominate the left and right frontwheels, the indices HL and HR denominating the left and right rearwheels. Consequently, (v_(VL)+v_(HL)) is the wheel speed sum of the sideof the left wheels, (v_(VR)+v_(HR)) being the wheel speed sum of theside of the right wheels of the vehicle. The absolute value of thedifference formed from these two sums provides thewheel-speed-difference value SD.

As long as difference SD lies below the speed-dependent thresholdillustrated in FIG. 3 and defined in this example by the straight linesth₁,th₂,th₃ there will be no management of the braking pressure build-upgradient of the wheel on the inside of the corner. Consequently,p=p_(max).

If the difference SD comes up to and exceeds threshold th₁ or th₂ orth₃, respectively, reduction of the braking pressure build-up gradientwill come on. The measure of reduction of the braking pressure gradientfrom the original value of p=p_(max) down to a value of p=p_(min),including even the possibility of p_(min) becoming zero, depends on theamount of the wheel-speed-difference value or on how much thesethresholds th₁ or th₂ or th₃, respectively, have been exceeded. Thehatched band “B” in FIG. 3 symbolically represents the range which maybe penetrated by the slip difference. The reduction of the brakingpressure build-up gradient depends on the “depth of penetration” intothis range.

FIG. 4 shows the pressure variation p as a function of time. Thestraight lines p₁(SD), p₂(SD), p₃(SD) and p_(n)(SD) are examples ofbraking pressure build-up gradients which may result. The highestgradient p₁(SD)=p_(max) represents the maximum gradient implied by thesystem. This gradient applies as long as the difference value SD liesunder the threshold values th₁,th₂,th₃. If, for instance, the differencevalue SD considerably exceeds the threshold values the inventive specialcontrol will adjust a braking pressure build-up gradient p₃ or such abraking pressure build-up gradient will be the result, respectively. Thehorizontal line in FIG. 4 symbolizes the limit value, namely gradientp_(n)(SD)=0.

The variation of the slip threshold in accordance with FIG. 3 depends onthe respective vehicle type. In the illustrated example it provedadvantageous with a vehicle speed of approximately 50 k.p.h. to switchover to the inventive special control as early as of a relatively smallslip difference of, e.g., SD=8 k.p.h. With lower and higher vehiclespeeds, the thresholds are higher. In the present example, the threshold(th₃) remains constant as of approximately 130 k.p.h.

What is claimed is:
 1. A method for improving the control behavior of ananti-lock control system of a four-wheel, two-axle vehicle, includingthe steps of: measuring rotational behavior of the vehicle wheels;deriving a vehicle reference speed; obtaining criteria for identifying acornering situation and the direction of cornering from the rotationalbehavior of the vehicle wheels; instead of a standard control mode,putting into operation a special control mode when cornering isidentified, said special control mode causing a reduction of brakingpressure on at least one wheel of the group consisting of the frontwheel on the inside of the corner and the rear wheel on the inside ofthe comer already in a partial braking range; and reducing the gradientof a braking pressure build-up on the at least one wheel in the partialbraking range when a threshold value of a wheel speed difference isexceeded, which difference is the difference between the wheel speedsums per vehicle side formed from the speeds of the two wheels of eachside of the vehicle, the reduction taking place in dependence on theextent by which the threshold value of the wheel speed difference isexceeded.
 2. A method as claimed in claim 1, wherein the threshold valueof the wheel speed difference varies in dependence on the vehiclereference speed.
 3. A method as claimed in claim 2, wherein the gradientof the braking pressure build-up varies between a maximum value whichapplies below the threshold value of the wheel speed difference and aminimum value which applies in case of a high difference.
 4. A method asclaimed in claim 2, wherein the gradient of the braking pressurebuild-up is adjusted by introducing the braking pressure via an inletvalve, actuated in a pulsed manner at a pulse-and-pulse-pause ratio, andby varying the pulse-and-pulse-pause ratio.